TruncateSum [sexpr,n]
truncates each Sum in sexpr to have at most n terms.
TruncateSum [sexpr,{m,n,…}]
truncates each multiple Sum in sexpr using the iterative specification {m,n,…}.
TruncateSum
TruncateSum [sexpr,n]
truncates each Sum in sexpr to have at most n terms.
TruncateSum [sexpr,{m,n,…}]
truncates each multiple Sum in sexpr using the iterative specification {m,n,…}.
Details and Options
- TruncateSum is typically used to truncate symbolic solutions involving infinite sums to finite sums, making it easy to numerically evaluate such approximations.
- The sum expression sexpr can have any combination of unevaluated and Inactive sums.
- TruncateSum will truncate large positive or negative summation limits according to the following:
-
, if, if
- The following options can be given:
-
Examples
open all close allBasic Examples (2)
Truncate an infinite sum to its first 12 terms:
Truncate an inactive sum:
Avoid activating the inactive sum:
Activate the result:
Scope (13)
Basic Uses (3)
Truncate an infinite sum to its first 10 terms:
Truncate an infinite sum to a sum with a symbolic upper limit:
For unevaluated sums, TruncateSum directly evaluates the truncated sum:
Finite Sums (3)
Truncate a finite sum:
Truncate a sum with symbolic upper limit:
Truncate a sum with symmetric upper and lower limits:
Infinite Sums (4)
Truncate an infinite sum:
Truncate a sum with doubly infinite limits:
Truncate a sum with –∞ as its lower limit:
Truncate a polynomial with infinite sum coefficients:
Multiple Sums (2)
Truncate a doubly infinite sum:
Specify the maximum number of total terms for each sum:
Truncate only the first sum:
Truncate only the second sum:
Truncate a double sum:
Specify the maximum number of individual terms:
Inactive Sums (1)
For inactive sums, TruncateSum tries to evaluate the truncated sum:
Use ActivateResult False to avoid activating the inactive sum:
Activate the result:
Options (2)
Applications (10)
Differential Equations (4)
Solve the Dirichlet problem for the wave equation on a finite interval:
The solution is an infinite trigonometric series:
Extract the first three terms from the Inactive sum:
Solve the Dirichlet problem for the wave equation in a rectangle:
The solution is a doubly infinite trigonometric series:
Extract a few terms from the Inactive sums:
Solve the Dirichlet problem for the heat equation on a finite interval:
The solution is a Fourier sine series:
Truncate the Inactive sum:
Solve the initial value problem for a Schrödinger equation with Dirichlet boundary conditions:
Define a family of partial sums of the solution:
For each k, uk satisfies the differential equation:
The boundary conditions are also satisfied:
The initial condition is only satisfied for u∞, but there is rapid convergence at t==2:
Difference Equations (1)
Solve a difference equation:
Extract the solution for :
Asymptotics (3)
Compute the power series expansion of around 0:
Obtain the first seven nonzero terms in the series:
Compute the power series expansion of around 1:
Obtain the first five terms in the series:
Truncate the series for the Hypergeometric1F1 function:
Compare the results with the built-in Hypergeometric1F1 function:
Inverse Laplace Transform (2)
Calculate the inverse Laplace transform of a function:
Truncate the sum and plot the result:
The inverse Laplace transform of this function is a piecewise function:
Truncate and plot the result:
Properties & Relations (4)
TruncateSum [expr,n] truncates each sum in the expression to have at most n terms:
TruncateSum truncates both limits of doubly infinite sums:
TruncateSum truncates only the lower limit of a sum from –∞ to a finite value:
TruncateSum activates all inactive sums in the expression:
ActivateResult False can be used to avoid activation:
Possible Issues (1)
The number of terms in the truncated sum is assumed to be an integer:
See Also
Sum Inactive DSolve Asymptotic RSolve InverseLaplaceTransform
Related Guides
History
Text
Wolfram Research (2024), TruncateSum, Wolfram Language function, https://reference.wolfram.com/language/ref/TruncateSum.html.
CMS
Wolfram Language. 2024. "TruncateSum." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/TruncateSum.html.
APA
Wolfram Language. (2024). TruncateSum. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/TruncateSum.html
BibTeX
@misc{reference.wolfram_2025_truncatesum, author="Wolfram Research", title="{TruncateSum}", year="2024", howpublished="\url{https://reference.wolfram.com/language/ref/TruncateSum.html}", note=[Accessed: 16-November-2025]}
BibLaTeX
@online{reference.wolfram_2025_truncatesum, organization={Wolfram Research}, title={TruncateSum}, year={2024}, url={https://reference.wolfram.com/language/ref/TruncateSum.html}, note=[Accessed: 16-November-2025]}